Thymidine analogues 5-trifluoromethyl-, 5-pentafluoroethyl- and 5-(heptafluoro-n-propyl)-2¡ä-deoxyuridines were synthesised and converted into the corresponding 5¡ä-triphosphates 1a¨Cc. Performing DNA polymerase-catalyzed primer extension reactions these modified nucleotides were incorporated into DNA to create perfluoroalkylated nucleic acids. Although single modified nucleotides were enzymatically incorporated and further elongated quite similar to the natural TTP, the enzymatic synthesis of multi-modified nucleic acids was initial only feasible with modifications at every fourth base. Nevertheless, as the effects of the modified dUTPs on DNA polymerases varied significantly with the used enzyme, Therminator DNA polymerase was proficient in incorporating 11 adjacent 5-trifluoromethyl-2¡ä-deoxyuridine moieties.